Hello! The two most common failure points are:
1) Vent valve, accessed at the right rear wheel-house.
2) Fuel tank. The tanks are plastic and often crack or fail to seal at the fuel pump flange. You can sometimes remove the back seat and see fuel stains from around the pump, or if you smoke test it, you would see smoke come from around the fuel pump area.
I have seen a few bad purge solenoids, but the Vent valve and fuel tank are (in my opinion) a little more common.
Technician/Shop Foreman
GM & Saab Certified, ASE Master Technician